This invention relates to the fabrication of electronic devices, and more particularly to improvement of process control in chemical-mechanical polishing (CMP) processes used in fabrication of semiconductor structures or ferromagnetic read/write heads.
In the field of semiconductor processing, CMP is a widely used technique for planarization of material and controlled removal of a layer of material from a stack of films on a substrate. A typical application of CMP is shown schematically in FIGS. 1A-1C. FIG. 1A illustrates a layer of material 1 deposited on an underlying layer of material 2 and overlying structures 3, prior to planarization of layer 1. As layer 1 is polished down, the top surfaces 3a of some of the structures are exposed, as shown in FIG. 1B (that is, a portion of the pattern defined by structures 3 is revealed). The CMP process continues until all the structures 3 have their top surfaces 3b exposed (FIG. 1C); this is the endpoint of the process.
A typical CMP apparatus 20 is shown in FIG. 2. A workpiece 10 (such as a silicon wafer with one or more layers deposited thereon) is held face down by a wafer carrier 11 and polished using a polishing pad 12 located on a polishing table 13; the workpiece is in contact with slurry 14. The wafer carrier 11 is rotated by a shaft 15 driven by a motor 16. The entire surface of the workpiece is polished by the polishing pad in the presence of the slurry.
In CMP processes generally, it is extremely important to stop the process at a desired predetermined location in the film or stack of films. Overpolishing (removing too much) of a film renders the workpiece unusable for further processing, thereby resulting in yield loss. Underpolishing (removing too little) of the film requires that the CMP process be repeated, which is tedious and costly. Tight tolerances are often maintained on the heights of structures 3; this in turn means that the endpoint must be precisely determined, and the process must then be promptly stopped. Moreover, as shown in FIGS. 1A-1C, the target surfaces for the process (that is, surfaces 3b) are often only a small fraction of the area of the whole wafer (as small as 2% in the case of NiFe structures with an overlying Al2O3 film, as in fabrication of magnetic write heads, for example in a P2 process). An endpoint detection and process control system for such a very low pattern density must detect the exposure of this small area, and do so with high accuracy and in real time.
In one conventional approach to the CMP endpoint detection problem, the thickness of the layer to be removed and the polishing rate are measured for each workpiece, in order to determine a desired polishing time; the CMP process is run for this length of time and then stopped. Since many different factors influence the polishing rate, and the polishing rate itself can change during a process, this approach is far from satisfactory.
More recently, CMP endpoint detection methods have been described which are based on slurry collection, sampling and analysis. A slurry sample 17, shown schematically in FIG. 2, is removed from the CMP apparatus by a slurry collector 21 and conducted through a delivery tube 21a into an analyzer unit 22. The analyzer unit determines whether the endpoint has been reached, based on changes in the composition of the slurry or in the concentration of a reaction product derived therefrom. A device for collecting slurry samples from the apparatus is described in U.S. Pat. No. 6,176,765, xe2x80x9cAccumulator for slurry sampling.xe2x80x9d As an example of an endpoint detection method based on slurry sample analysis, U.S. Pat. No. 6,440,263, xe2x80x9cIndirect endpoint detection by chemical reaction and chemiluminescence,xe2x80x9d describes analysis of the concentration of ammonia gas produced in the slurry when a silicon nitride film is exposed by polishing and chemically reacts with the slurry. This particular technique may be employed in polishing a variety of structures. For example, U.S. Pat. No. 6,291,351, xe2x80x9cEnd-point detection in chemical-mechanical polishing of cloisonnxc3xa9 structures,xe2x80x9d discloses a CMP Al2O3 film removal process in which a nitride film is deposited on top of NiFe structures, so that ammonia production indicates that the desired interface between the NiFe and Al2O3 has been reached. The disclosures of all three of the above-noted patents are incorporated herein by reference.
A variety of analysis techniques have been suggested in addition to the chemiluminescence approach described in the ""263 patent. These include atomic emission spectroscopy and mass spectroscopy for elemental analysis of polishing residues dissolved in the slurry, as is understood by those skilled in the art. All of the various detection and analytical techniques face the same difficulty: delivering accurate, reliable performance in a manufacturing environment.
In order to be useful in a manufacturing environment where processing typically is performed 24 hours a day, a CMP slurry-sampling endpoint detection apparatus must satisfy two requirements: (1) The apparatus must operate continuously, with minimal need for operator supervision and minimal downtime for cleaning or maintenance; this is sometimes termed the xe2x80x9crobustnessxe2x80x9d requirement. (2) The apparatus must monitor and control film removal in situ and in real time (with process control in response to endpoint within 1 second), even when the pattern density is as small as 2%; this is sometimes termed the xe2x80x9csensitivityxe2x80x9d requirement. Since endpoint detection depends on analysis of slurry samples, the slurry must be sampled throughout the polishing process. However, sampling is generally conducted only while slurry is delivered to the polishing apparatus, and is interrupted when the polishing process stops. This means that the slurry delivery tube 21a often becomes clogged with stagnated or partially dried slurry during periods between polishing processes. In addition, the sampled slurry must be flushed out of the endpoint detection apparatus before the next workpiece is polished, so that newly-sampled slurry is not contaminated by slurry from the previous process. This problem is particularly acute when the pattern density is small; in some instances the polishing residues in the slurry from the target surfaces have concentrations below 1 ppm.
There remains a need for a slurry-sampling CMP endpoint detection apparatus which meets the robustness and sensitivity requirements, and in particular is suitable for detecting process endpoint in wafers with a low pattern density in a manufacturing environment.
The present invention addresses the above-described need by providing a CMP system and method for transporting slurry through a tube to an endpoint detection apparatus while a CMP apparatus is performing a polishing operation, and for flushing the slurry sample transport tube while the apparatus is not performing a polishing operation.
According to a first aspect of the invention, a method of performing CMP on a workpiece includes providing a slurry sampling tube for conducting slurry from the CMP apparatus to the endpoint detection apparatus; providing a slurry flushing apparatus for flushing slurry from the sampling tube; pumping slurry through the sampling tube in a slurry flow direction from the CMP apparatus into the endpoint detection apparatus while a polishing operation is in progress; and flushing the sampling tube while a polishing operation is not in progress. The flushing of the sampling tube is commenced in accordance with a control signal from the endpoint detection apparatus terminating the polishing operation; the flushing is terminated in accordance with a starting signal for the next polishing operation. The flushing apparatus includes a first flow control valve, and the flushing operation includes opening the valve to commence flushing when the polishing operation is terminated, and closing the valve to terminate flushing when a next polishing operation is commenced. The flushing apparatus also includes a second flow control valve (for example, a needle valve) for controlling a flow of water to the first control valve. During the flushing operation, a first portion of the water flow is pumped into the endpoint detection apparatus while a second portion of the water flow flushes the sampling tube in a flushing direction opposite the slurry flow direction. The slurry is collected from the CMP apparatus by a slurry collector, which also is flushed while the polishing operation is not in progress. The pumping is advantageously performed by a peristaltic pump which continuously pumps slurry and/or water.
According to a second aspect of the invention, a CMP system is provided which includes a slurry transport and flushing apparatus. This apparatus has a slurry collector for collecting slurry from the CMP apparatus during a polishing operation; a slurry sampling tube connecting the slurry collector and an endpoint detection apparatus; a first flow control valve, connected to the slurry sampling tube, for controlling a flow of water into the slurry sampling tube to flush slurry from the sampling tube while the polishing operation is not in progress; and a second flow control valve, connected to said first control valve by a flushing tube, for controlling the flow of water to the first control valve. The endpoint detection apparatus includes a pump (preferably a peristaltic pump) for pumping slurry through the slurry sampling tube during the polishing operation. The flow of water is effective to flush the sampling tube while the polishing operation is not in progress; a first portion of this flow is pumped into the endpoint detection apparatus, and a second portion of this flow flushes the tube in a direction opposite that of slurry flow during a polishing operation. The first flow control valve is opened when the polishing operation is terminated in accordance with an endpoint signal from the endpoint detection apparatus, and is closed when a next polishing operation is commenced in accordance with a starting signal. The first flow control valve is characterized as normally-open/active-closed, so that the valve is closed by an electrical signal in response to an electrical starting signal for starting a polishing operation.